1. Field of the Invention
The present invention relates to an injection syringe comprising a cylinder having a longitudinal axis extending between opposite ends of the cylinder, a piston guided in the cylinder and dividing the cylinder into two cylinder chambers respectively facing the opposite cylinder ends, a piston rod preferably having a length exceeding that of the cylinder and coupled to the piston, the piston rod extending in one cylinder chamber and through one cylinder end, a coupling arrangement at a cylinder end opposite to the one end for coupling an injection needle held by the coupling arrangement to the cylinder, the piston and the opposite cylinder end defining the other cylinder chamber, and a safeguard against reuse of the injection syringe, the safeguard comprising an opening member between the piston and the piston rod.
2. Description of the Prior Art
A non-reusable syringe of this type is disclosed in British patent No. 2,207,054. The opening member is formed by a stopper connected to the piston rod, and when the injection fluid is pressed out by the piston, the stopper must be pushed through to connect the chamber containing the fluid to the ambient atmosphere so that no further suction is applied to the medicament. The end of the piston may also be equipped with a deformable sealing membrane which is pressed against an opening spike when pressure is applied to the piston to squeeze the injection fluid out. This is designed to damage the sealing membrane and prevent further fluid from being drawn up by the syringe. This structure has the disadvantage that even a slight operating error, for example while the injection fluid is drawn up, may damage the syringe and make it useless.
French patent No. 2,606,643 discloses a somewhat similar syringe and its safeguard against re-use at an end wall of the cylinder facing the needle comprises an opening member connected to the cylinder and pointing towards the piston. A sealing membrane in the vicinity of the piston is associated with the opening membrane. When the piston is moved forward by the piston rod into abutment with the end wall of the cylinder, the sealing membrane will be opened and no further injection fluid can be drawn up into the cylinder so that the syringe cannot be re-used. The disadvantage is that injection fluid could be drawn up in succession several times and administered to successive patients if the cylinder is not completely emptied, i.e. if the piston is not moved forward into abutment with the cylinder end wall. Therefore, re-use cannot be reliably prevented.
Other types of non-reusable syringes are also known. To avoid re-use, Published British Patent Application No. 2,195,537 proposes to surround the injection needle with a protective sheath. When this is removed for use of the syringe, one can see that the protective seal has been broken.
It is also known to provide seals formed by membranes between the needle and the cylinder of the syringe. These are only pierced immediately before the syringe is used so that the medicament is hermetically sealed in the cylinder until the syringe is actually used. The disadvantage of these syringes is that more medicament may be sucked into the cylinder when medicament has been discharged therefrom by operating the piston so that the syringe may be used a second time. In other words, such syringes are re-usable, which constitutes a danger particularly in countries with little awareness of hygiene, and may lead to an epidemic spread of contagious diseases, such as AIDS.